Acoustic waves are dispersive, when their phase and group velocities are frequency dependent. This dispersion may be caused by elastic property gradient (e.g. in composite materials) or by guided propagation (e.g. propagation in plates, rods and fibers). The attenuation of acoustic wave can be caused by various phenomena, including scattering by the microstructure of the sample under test, thermoelastic or heating effects, magnetoelastic loss effects in ferromagnetic materials and others. The measurement of dispersion, i.e. the determination of the phase and group velocities versus frequency, and of the attenuation can provide useful information on the specimens under test and its material properties. Acoustic dispersion measurements have been used to characterize subsurface anomalies, to estimate physical property gradients, to evaluate the thickness, the elastic properties or the microstructure of thin films, and to characterize composite materials. Attenuation measurements have been used to characterize the microstructure and various physical properties. See Physical Acoustics, Editors, W. P. Mason and R. N. Thurston, Accademic Press., N.Y., Vol. 12, Chapter 5 (1976), pp 277-374, "Ultrasonic velocity and attenuation measurement methods with scientific and industrial applicarions" by E. P. Papadakis; Ultrasonics, Mar. 1985, pp 55-62, "Laser generation of convergent acoustic waves for material inspection" by P Cielo et al. and Applied Physics Letters, Vol. 52, No. 14, 1987, pp 1066-1068, "Estimation of the thickness of thin metal sheets using laser generated ultrasound", by R. J. Dewhurst et al.
In most cases, acoustic waves are generated and detected using piezoelectric transducers either in direct contact with the sample or coupled to it with a liquid couplant. However in the cases where samples are at elevated temperatures or in motion etc., laser-ultrasonics is often employed which uses lasers to generate and detect ultrasound, without contact and at distance. See Canadian Journal of Physics, Vol. 64, NO. 9, 1986, pp 1247-1264, "Mechanisms of pulsed photoacoustic generation" by D. A. Hutchins and IEEE Transactions on Ultrasonics, Ferroelectrics, Frequency Control, Vol. UFFC-33, No. 5, 1986, pp 485-489, "Optical detection of ultrasound" by J-P Monchalin.
Historically, measurement techniques for the determination of the attenuation and of the phase and group velocities have evolved from discrete-frequency methods, such as the .pi.-point phase technique or the tone-burst method, to a frequency-wideband method based on spectral analysis. The .pi.-point phase technique and the tone-burst methods are harmonic methods, i.e. the acoustic waves are generated and detected at a single frequency, and the attenuation and the phase and group velocities are measured at this single frequency. A drawback of these harmonic methods is the necessity of a harmonic acoustic source of easily variable frequency. The spectral analysis method has been reviewed in Journal of Applied Physics Vol. 49, No. 8, 1978, pp 4320-4327 "On the determination of phase and group velocities of dispersive waves in solids" by Sachse et al. U.S. Pat. No. 4,372,163 Feb. 8, 1983 (Ahlberg et al) also describes this technique. The spectral analysis method using Fourier transform has the advantage of providing a broad frequency coverage from the analysis of the spectrum of short pulsed acoustic waves. When several propagating acoustic modes are mixed, as it is the case in waveguides (plates, rods, fibers) for example, the spectral analysis method is not applicable because the spectrum of the measured acoustic wave is an average of the spectra of the mixed waves. The information on each mode is thus mixed both in frequency and time. As a result, there is at least one case where these classical methods cannot be used: when a monochromatic acoustic source is difficult to realize, the harmonic methods are not usable, and when several propagating acoustic modes are mixed, the spectral analysis method is not applicable. The present invention is a solution for such cases and processes broadband acoustic waves. The mixed modes may be identified after narrow-band filtering and separated before processing.